by Kalid Azad
1.1 The Problem of Indoor Positioning
Currently, there is no commonly agreed-upon way to determine position indoors. In contrast, GPS (Global Positioning System) works well outdoors and is almost universally used. However, its signals are weak enough to be blocked by the walls of a building, and thus is not available for indoor applications. Various methods exist for indoor positioning, but all must balance flexibility, cost, and accuracy.
The applications of indoor positioning are many: indoor robots, inventory tracking, security, and location-finding, for instance. Currently, the main problem lies not with the need for applications, but with the implementation.
The challenge occurs precisely because of the tradeoff mentioned above. It is natural that a more accurate system should cost more than a less accurate one, or have some other disadvantage. GPS, for instance, cost hundreds of millions of dollars to develop, but is extremely accurate and flexible - it can be used anywhere on Earth. Various issues arise when designing a positioning system, such as clock synchronization, propagation delay timing, signal absorbtion, reflection and interference, and the challenge of creating custom hardware.
1.2 A Possible Solution
This paper introduces an extension to an existing indoor positioning system that uses a wireless network. The system, developed by researchers at Microsoft, works by having a client visit various locations and record the signals seen at each one (its ``profile"). After collecting many profiles, another client can then wander around, comparing what their current profile to the prerecorded ones. The nearest match is probably the user's location. My key contribution to this scheme is the use of a directional antenna to improve a profile's quality. The goal is that a better profile should lead to improved accuracy.
1.3 Overview of Paper: Executive Summary
We begin by overviewing previous attempts at indoor positioning, which interestingly share similarities to a historical search for a positioning system. All systems except for positioning via phase differences and wireless networking are eliminated, mostly for reasons of cost, complexity, or accuracy. The phase difference system was attempted, but found to be too complex to complete in the time given; the wireless system was then selected, and was actually the better choice in hindsight. The details of my approach and implementation are given (including the choice of a can-waveguide antenna), followed by the results: meter-level accuracy can be obtained on average. One problem, however, is that occasionally the system's first location choice is inaccurate by a significant amount, whereas the second choice is correct. Various methods are proposed to improve upon the system, followed by concluding remarks.
* Cheap! Wireless card $80, homebrew antenna $5-10 dollars, N pigtail cable $20-40.
* Uses existing technology. The most complex part, the card, is easily bought. The antenna is easy to build, and I wrote some software so you don't have to! Intended for use indoors, where a wireless network is easily installed (if not already).
* Specify your own level of resolution (more points recorded = better resolution)
* Easy setup: Walk around, record some locations, and you are done. Not much external hardware (access points, wireless card, that's it).
* Need to visit locations beforehand. This isn't so bad, since it is indoors, and you aren't spelunking in unknown caves or exploring the Bermuda triangle. Use GPS for that . It only takes about 10-20 seconds to record a point for decent precision. This is also better than the brute-force IR ActiveBadge system, which makes you install a receiver at every point! However, it still might take a day to setup a large office building. Still, that's not bad.
* Not good for frequently changing environments. If walls/floors/furniture are shifting often, signal strengths will vary. This probably isn't likely.
* Precision. The longer you wait at a point, the better (I average the signals received). Again, 10 seconds is all you need for resolution of 2-3 meters. This is far better than GPS was before it was unscrambled it for civilian use.
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